In current practice, it is usual, when starting an internal combustion engine of a vehicle for the starter to be cut off, i.e. for the action whereby the starter itself drives the engine, to be terminated, under the direct control of the driver of the vehicle, who for this purpose releases the ignition key from its "start" position when the engine gives out a characteristic sound. However, the prevailing tendency to make engines more and more quiet makes it increasingly difficult for the driver to detect this characteristic sound, and therefore difficult to know when to release the ignition key so as to disengage the starter. This results in the application of unnecessary, and severe, forces between the starter and the engine.
Numerous devices are already known for cutting off (i.e. interrupting the power supply to) a motor vehicle starter when the engine has been successfully started and is sufficiently autonomous to reach its slow running mode by itself. The best of these devices, in terms of performance generally, make use of an analysis of the fluctuations which occur in the power supply voltage to the starter. These fluctuations are due to variations in the current taken by the starter motor during the compression strokes of the internal combustion engine before the latter has been fully started.
It is well known that at the end of the period during which the engine is being driven by the starter, and during the first explosions in the engine, engine speed rises rapidly, so that starter cut-off should take place not only very rapidly, but also at exactly the right moment.
In order to obtain rapid cut-off of the power supply to the starter as soon as the engine has been driven by a sufficient amount, it has previously been proposed, in French patent application No. 96 11792 in the name of Valeo Equipements Electriques Moteur, to provide a cut-off strategy whereby an inspection time period is commenced for each fresh fluctuation, and the power supply to the starter is interrupted when no extreme (maximum or minimum) value has been detected during this inspection time period. The duration of the inspection time period is for example proportional to the duration of the last preceding fluctuation. However, that strategy does not enable acyclic fluctuation to be controlled.
Such acyclic fluctuation is shown in FIGS. 1 and 2 of the accompanying drawings, to which reference is now made. Each of FIGS. 1 and 2 shows a waveform of the starter voltage, plotted against time, at the commencement of the operation of the starter.
The acyclic effect shown in FIG. 1 occurs at the very beginning of the operation of the starter, with a peak C.sub.1 at the beginning of the first compression stroke of the engine being hidden in the rising front of the second fluctuation, which has a second peak C.sub.2. In other words, in the first two compression strokes, the corresponding fluctuations are substantially coincident with each other. Thus, analysis of the time period T.sub.1 for reaching the first detectable peak (C.sub.2) in the voltage signal is erroneous, and the inspection time period D.sub.1 which is determined, on the basis of the length of the period T.sub.1, for the next following cycle of the engine, after the peak C.sub.2, is much longer than necessary. Consequently, if starting of the engine occurs at this time, the decision to de-energise the starter, taken at the end of the inspection time period, is late. This substantially reduces the advantages expected from the electronic control system that gives automatic cut-off, such as reduction in wear, reduction in free-wheel noise, and so on.
The acyclic effect illustrated in FIG. 2 occurs during the subsequent phase in which the engine is being driven by the starter, in cases where starting is proving difficult. An explosion which is not followed by further explosions again gives rise to substantial coincidence of fluctuations, so that two successive voltage peaks are brought too close together, as illustrated in FIG. 2 by the peaks C.sub.3 and C.sub.4. As a result, the first of these peaks, i.e. C.sub.3, will tend not to be detected during the corresponding inspection time period D.sub.2.
In this case therefore, there is a danger that the close proximity of two explosions to each other will be interpreted by the electronic control system as an indication that the engine has started. The starter will then be stopped prematurely, before the engine has itself started properly, so that the driver then has to commence the starting operation again from the beginning.
In addition, the time T.sub.3 used for determining the next inspection time period is also in error.